Some lighting circuit such as those used for lights on automobiles traditionally used conventional light bulbs. For example, the taillight, park-light and indicator-lights provided at the rear of a motor vehicle would include a number of light bulbs in parallel circuits powered by a DC-powered supply from the vehicle.
There are advantages in utilizing solid-state devices for such lights as solid-state devices have significantly longer life expectancies and reduce the need for bulb replacement and risk of failure of the lights.
In using solid-state lighting devices, an array of such devices may be provided such as an array of LEDs. The LEDs are generally driven at constant current to ensure the correct illumination from the LEDs and each of the devices will have a power demand and voltage drop across the device. With a single LED being relatively small and its total light output being insufficient for such lights, it is typical to supply an array of such LEDs to form a single light.
The difficulty with incorporating solid-state devices such as LEDs in automotive uses is that the power supply from a vehicle may fluctuate to some considerable degree. For example, an automotive power supply may provide variable voltages between 6 and 26 volts. Different voltages will be available depending on different states of the battery or generator in the vehicle at any particular time or the instantaneous load drawn from the overall power supply.
With the LEDs operating on constant currents, it is typical to require some kind of power converter and regulator to ensure that the LEDs are driven at the appropriate current and with sufficient voltage. Usually these would require ensuring that the lighting array can operate at the lower end of the likely range of supply voltages and if the voltage is higher, the excess power may be wasted and requires dissipation as heat.
Different circuits have been proposed to provide greater efficiencies. For example, linear regulator circuits can provide the necessary control over the power supply although are relatively inefficient.
An alternative is a switching power supply but such power supplies can cause high electromagnetic interference through the high-speed switching of the device.